Hard disc drives are typically utilized as primary data storage devices in modern computer systems. Such disc drives record digital data on a plurality of circular, concentric tracks on the surfaces of one or more rigid discs. The discs are axially aligned and mounted to a hub of a spindle motor for rotation at a constant high speed.
Data are stored on and retrieved from the tracks using an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by a rotary actuator assembly. Each head typically comprises an electromagnetic transducer carried on an air bearing slider, which is supported over the corresponding disc surface on an air bearing established by air currents set up by rotation of the disc.
The position of the heads with respect to the tracks is controlled by a closed loop digital servo system. Servo patterns used to define the location of the heads relative to the disc surface are prerecorded on the discs during the disc drive manufacturing process. These servo patterns can be recorded exclusively on one surface of one disc and continuously read (as in a dedicated servo system), or can be interspersed among the various discs so that each track includes both servo and data blocks (as in an embedded servo system), the data blocks being used to store user data provided from a host computer in which the disc drive is mounted.
The data blocks, also sometimes referred to as "sectors", include a number of leading control fields which enable read channel circuitry of the disc drive to properly decode the user data information stored in each data block. Such control fields typically include phase-locked oscillator (PLO) and training fields to enable the read channel to set various gain and timing parameters before recovering the user data. Synchronization fields enable the channel to correctly detect the beginning of the user data stored in the data block. Thus, the control fields optimize the operation of the channel for each data access operation.
As will be recognized, continual advancements in the art have led to greater levels of data storage and transfer rate capabilities in successive generations of drives. One such advancement is the implementation of magneto-resistive (MR) heads, which utilize magneto-resistive elements having changed electrical resistances in the presence of magnetic fields of a selected orientation. Data previously stored to a data block can be detected by changes in voltage across an MR element as a read bias current is passed therethrough.
The use of MR heads has allowed disc drive designers to bring the heads ever closer to the disc surfaces. This provides the advantage of greater data recording densities, but introduces a greater likelihood of distortion in the readback signals generated by the heads due to thermal interaction between the heads and the discs. More particularly, thermal asperities (TAs) are distortion events in the head readback signals caused by changes in the temperature of the heads as the heads fly over the surfaces of the discs. TAs typically result from actual physical contact between the head and a contaminating particle on the disc (or a localized "hill" on the disc), but can also be induced as a result of a change between the relative flying height of the head as the head passes over "hills" and "valleys" on an irregular disc surface.
TAs found in disc drives using currently available media are of a size which can span a significant number of bytes; for example, in a disc drive having a data transfer rate of 200 megabits per second (Mbits/sec), uncompensated thermal asperities can typically last from 2 to 5 microseconds, distorting from about 50 to 125 bytes of data. Further, it will be recognized that TAs can grow over time due to factors such as contamination and corrosion of the disc surfaces, which can significantly degrade the capabilities of a disc drive to reliably store and retrieve user data over the operational life of the drive. Localized media anomalies can also cause problems in the storage and recovery of data from the discs, preventing a disc drive from recovering data previously stored to a given data block.
To compensate for the effects of TAs and localized media anomalies, as well as other types of various anomalous conditions, a data block format has been proposed which utilizes redundant synchronization fields to improve the ability of a disc drive to recover previously stored data. More particularly, first and second synchronization (sync) fields are provided in each data block so that the data stored in the data block can still be recovered when an anomalous condition prevents the read channel from correctly decoding one of the two sync fields. More particularly, the distance separating the two sync fields in a given data block is selected to be of sufficient length so that a TA coincident with one of the sync fields does not interfere with the remaining sync field.
Alternative formatting methodologies can be used to separate the two sync fields in each data block. In one approach, a second PLO field is written in the space between the two sync fields. In another approach, a first data field is disposed between the two sync fields, with a portion of the data being stored in this first data field and the rest of the data stored in a second data field following the second sync field. In the latter approach, the amount of data stored in the first data field is selected to be within the error correcting capabilities of the read channel so that this data can be reconstructed from error correction code symbols appended to the data, should the read channel be unable to correctly detect the first sync field.
Although the redundant sync field data field format has been found to greatly enhance the ability of a disc drive to recover previously stored data, problems have been found to arise in certain circumstances, such as when the same sync pattern is used in both of the sync fields or when a redundant sync field data field is split by a servo block. Accordingly, improvements are needed to facilitate further advancements in disc drive performance, and it is to this end that the present invention is directed.